This invention relates to a voltage-to-frequency converter circuit which is chiefly used in an electronic wattmeter.
In an electronic wattmeter, the load voltage and consumption current of a distribution line are multiplied by means of a multiplier unit to create a voltage signal proportional to instantaneous power, the voltage signal is applied to a voltage-to-frequency converter circuit to provide, therefrom, a rectangular wave signal of a frequency proportional to the voltage signal or to the instantaneous power. The rectangular wave signals are counted by a counter circuit to calculate electric energy, and the electric energy is integrated and displayed by a display unit.
Here, in order to raise the precision of a wattmeter, the voltage-to-frequency converter circuit is required to have the linearity of the output frequency to the input voltage in the very wide voltage region of from several millivolts to several tens of volts.
With the voltage-to-frequency converter circuit of this type, however, a malfunction often occurs due to the operation delays of circuit elements in a high frequency region, so that the desired linearity is spoiled. This will be described in detail with reference to the prior-art voltage-to-frequency converter circuit shown in FIG. 1.
Referring to FIG. 1, numerals 10 and 10 designate a pair of input terminals. The input voltage signal being proportional to the instantaneous power from the multiplier unit as described above is received across the input terminals 10 and 10. By means of an input circuit 14, this signal is converted a pair of D.C. voltage signals. A positive voltage signal (ep) and (en) whose absolute values are equal and whose polarities are opposite. The input circuit 14 is comprised of an operational amplifier 11 and resistors 12, 13. Inverting switching circuit 15 is a traditional analog switch, resistor 16 connects input circuit 14, to a standard integrator circuit 17 comprising a capacitor 19 in the negative feedback circuit of an operational amplifier 18. An output circuit 20 is composed of an operational amplifier 21 and resistors 22, 23, to form a hysteresis comparator. More specifically, the output circuit 20 produces, as its output, a rectangular wave signal a ((B) in FIG. 2) which is inverted each time the output voltage E.sub.O of the integrator 17 reaches a predetermined upper limit threshold voltage value +V.sub.R or lower limit threshold voltage value -V.sub.R shown at (A) in FIG. 2. The rectangular wave signal a is also used as a signal for driving the inverting switching circuit 15.
Next, the operation of the above circuit arrangement will be explained. When the inverting switching circuit 15 is in the operating state illustrated in FIG. 1, the D.C. voltage signal ep is applied to the minus input portion 24 of the integrator 17, and the capacitor 19 is charged. Thus, integration is performed, and the output voltage E.sub.O of the integrator 17 lowers as indicated by a rightwardly-descending straight line L.sub.2 in (A) of FIG. 2. When the output voltage E.sub.O has reached the predetermined lower limit value -V.sub.R, the rectangular wave signal a provided from the hysteresis comparator 20 becomes logic level "0" as shown in (B) of FIG. 2. This rectangular wave signal a actuates the inverting switching circuit 15 which is then inverted, and the D.C. voltage signal en is applied to the minus input portion 24 of the integrator 17. Thus, charges in the capacitor 19 are discharged, and the output voltage E.sub.O of the integration circuit 17 rises as indicated by a rightwardly-ascending straight line L.sub.1 in (A) of FIG. 2. When this output voltage E.sub. O has reached the predetermined upper limit value +V.sub.R, the rectangular wave signal a from the hysteresis comparator 20 in FIG. 1 becomes logic level "1" as shown in (B) of FIG. 2. Then, the inverting switching circuit 15 is actuated and inverted again, back into the original state by this rectangular wave signal a.
The integral voltage E.sub.O in (A) of FIG. 2 thus obtained comes to have a steeper gradient as the input voltage signal e in FIG. 1 is greater, with the result that a period T indicated in (A) of FIG. 2 becomes shorter. Since this period T is identical to the period of the rectangular wave signal a in (B) of FIG. 2, theoretically the frequency of the rectangular wave signals a is proportional to the magnitude of the input voltage signal e.
In actuality, however, there are the so-called delays of circuit elements such as the delay between the input and output of the hysteresis comparator 20 in FIG. 1 and the change-over time of the inverting switching circuit 15, so that an overshoot voltage E.sub.r and an undershoot voltage E.sub.r shown in (A) of FIG. 2 appear. Thus, the period T of the integral voltage E.sub.O becomes longer than the true value T.sub.O by 4 t.sub.d. Therefore, the foregoing proportional relationship between the input voltage signal e and the frequency of the rectangular wave signals a, i.e. the linearity, is spoiled. This will be explained below by the use of mathematical expressions:
Letting R.sub.3 denote the resistance of the resistor 16 and C denote the capacitance of the capacitor 19, the quantity of charges which are stored by the overshoot is: EQU E.sub.r .multidot.C=(ep/R.sub.3).multidot.t.sub.d .thrfore.E.sub.r =(ep/C R.sub.3).multidot.t.sub.d ( 1)
Accordingly, the quantity of charges in one period T becomes: ##EQU1## The frequency f becomes: ##EQU2## At t.sub.d =0, namely, in the absence of the overshoot, f=(ep/4 C R.sub.3) holds and it is proportional to the input voltage ep (=e). At t.sub.d .noteq.0, however, the aforementioned proportional relationship does not hold.